Anomalous Microwave Excess (AME)

Anomalous Microwave Emission (AME) is a diffuse emission component detected across the sky in the 10–60 GHz frequency range that cannot be accounted for by the three classic Galactic foregrounds — synchrotron radiation, free-free emission, and thermal dust emission. First identified as an unexpected correlation between microwave and far-infrared dust emission in the late 1990s, AME is now understood to arise primarily from spinning dust grains: small carbonaceous or silicate particles with electric dipole moments that emit electric-dipole radiation as they rotate at gigahertz frequencies. While the spinning dust model broadly fits the spectral shape, the spatial distribution, amplitude, and spectral peak of AME show variations across different environments — molecular clouds, HII regions, diffuse ISM, and dark clouds — that are not fully reproduced by any single spinning dust model. These variations introduce systematic uncertainties in CMB foreground separation that propagate into cosmological parameter estimates.

Successive Collision Theory provides a physical framework for understanding the environmental variation in AME through the angular momentum inheritance mechanism applied to interstellar dust dynamics. In SCT, the interstellar medium of the Milky Way formed from collision debris with a specific large-scale angular momentum field that determined the organization of gas, dust, and magnetic fields across the Galactic disk. The spinning dust grain rotation rates, which set the AME spectral peak frequency, depend on the local radiation field intensity, gas density, and grain-gas coupling — all of which are modulated by the angular momentum structure of the interstellar environment. Regions with higher angular momentum density from the inherited collision field have more organized magnetic field structures and higher gas velocity dispersions, which alter the equilibrium rotation rates of spinning dust grains and shift the AME spectral peak. The observed environment-dependent variation in AME properties therefore traces the angular momentum gradient of the local ISM established by the collision debris.

The pre-existing stellar populations from the colliding pockets contributed dust grains with a wider range of compositions, sizes, and surface electric dipole moment densities than grains produced exclusively from post-collision stellar evolution. Pre-existing carbonaceous and polycyclic aromatic hydrocarbon grains from prior stellar generations mix with post-collision dust, producing a grain size and composition distribution that differs subtly from a single-epoch stellar population synthesis. This altered grain distribution produces AME spectral shapes and amplitudes that deviate from models calibrated on pure post-collision stellar dust, explaining the residuals between observations and spinning dust models in high-latitude diffuse regions where the pre-collision grain contribution is relatively less diluted by subsequent stellar processing.